The present invention relates to a spark plug to be used for internal combustion engines.
Conventionally, there has been known a spark plug 100 formed as shown in FIG. 17. In this spark plug 100, an insulator 102 and a metallic shell 103 are placed concentrically with each other outside a cylindrical center electrode 101, and one end of ground electrode 105 is connected to the metallic shell 103. The other end of the ground electrode 105 is bent sideways so that a front side face of the ground electrode 105 is opposed to a front end face of the center electrode 101, where a spark discharge gap g is formed between the ground electrode 105 and the center electrode 101. The spark plug 100 of such a structure is fitted to, for example, a cylinder head of an internal combustion engine so as to be used as an ignition source for mixed air fed to the combustion chamber.
In order that the air-fuel mixture within the engine cylinder is ignited by using the spark plug 100 and thus burned, it is important not only to securely ignite the air-fuel mixture at near the spark discharge gap g but also to promptly propagate the flame from a pilot burner to the air-fuel mixture within the cylinder. For this purpose, the air-fuel mixture must be smoothly fed to around the spark discharge gap g, whereas the ground electrode 105 presents beside the center electrode 101 is likely to be obstructive to the air-fuel mixture flow or the flame propagation. For example, if a space K formed beside the center electrode 101 with the ground electrode 105 is too small, the air-fuel mixture would less flow into the space K, which could cause an obstacle for the ignition. Also, too small a space K would hinder the outflow of the flame generated by the ignition, which would make it more likely to occur that heat is deprived by the contact with the ground electrode 105 to hinder the flame propagation, i.e. a flame quenching effect. These phenomena can make a cause of deterioration in the ignitability of the spark plug.